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osteolytic, and secretion of growth factors including epidermal growth factors, bone morphogenic proteins
and platelet derived growth factors from tumour cells increase osteoblast activity, resulting in the
[13]
production of mixed lesions which can be observed in a subset of patients (reviewed in ).

Breast cancer bone metastasis is a complex process involving many signalling pathways and cell types, and
there are potentially variations in mechanisms that drive metastasis from different molecular sub-types.
Because of this complexity, a variety of in vivo mouse models has been developed to allow researchers to
investigate specific aspects of this disease process.

MOUSE MODELS
Human breast cancer xenograft models
To facilitate growth of human breast cancer cells in a different species, such as mouse, it is essential to use
an immunodeficient host. The majority of human breast cancer cells lines that are either trophic to the bone
environment or form overt tumours following injection directly into bone grow readily in nude mice that
have been bred on a BALB/c or MF1 background [Table 1] [14-19] . These mice have no thymus and therefore
cannot generate mature T lymphocytes and are unable to mount many types of adaptive immune responses
including graft rejection . Some, more difficult to culture cell lines as well as patient derived xenografts
[20]
(PDXs) will only graft/grow in more severely immunocompromised mice. For these models, NOD SCID
mice that have severe immunodeficiency affecting T and B lymphocyte development as well as having
reduced NK cells, macrophages and granulocytes numbers and functionality can be utilized [21-23] . There has
been a recent trend amongst researchers to move towards the use of even more severely
immunocompromised, NOD SCID ϒ (NSG) mice, for grafting PDXs, as these animals are deficient in
multiple cytokine signalling pathways as well as those seen in NOD SCID mice increasing the likelihood of
successful tumour engraftment [23,24] . Because immune response has profound effects on tumorigenesis and
the activity of many anti-cancer agents [Table 2], data obtained from these models must be interpreted
cautiously.

Immortalised breast cancer cell lines
Injecting human bone trophic breast cancer cell lines into the blood stream of young (4-8 weeks old) mice is
the most commonly used model for generating bone metastases in the laboratory. This method is useful for
investigating tumour cell homing, colonisation, metastatic outgrowth and associated interactions with the
bone microenvironment as well as therapeutics. Injection of human triple negative, MDA-MB-231, or ER-
positive, MCF7, cells into the left cardiac ventricle or carotid artery in appropriate mouse strains results in
tumour growth in the long bones spine and jaw [Table 1] [6,14,16,17,25] . This method works particularly well with
MDA-MB-231 cells from which osteolytic tumours develop in 60%-90% of animals 2-4 weeks following
injection. Bone metastasis from MCF7 cells form mixed (lytic/blastic) lesions, however these occur less
[26]
frequently, over an extended time period (20-25 weeks) and often require oestradiol supplementation .
Data showing bone metastases from intra-cardiac injection of other human breast cancer cell lines are
lacking, suggesting that this is an infrequent event. The high propensity of MDA-MB-231 cells to
metastasise to bone following intra-cardiac injection has resulted in this becoming the model of choice for
many researchers, but this model is not without its limitations:

· The majority of bone metastasis from breast cancers, in humans, occur from ER-positive rather than ER-
negative disease represented by MDA-MB-231 cells.

· Bone metastasis is associated with tumour cells, from the primary site, homing to specific metastatic niches
in bone and undergoing long periods of dormancy in these sites before outgrowth into overt metastasis. The